Just a slight departure from the thread topic.
In West Texas:
if 2 is the desirable minimum
then 4 is good
and 8 is what you really want.
In my wet/dry biological filtration process the flow is through
Then through, in parallel, ceramic cylindrical toroids and ceramic toroids which are placed under the bioballs in the 1st chamber of the sump;
Then through sintered glass placed along the underflow from the 1st chamber of the sump to the 2nd chamber of the sump.
In addition I have two limestone rocks placed in the 2nd chamber of my sump.
These rocks, in addition to providing some hardness to the tank water, also have a significant quantity of crevices and cavities which provide significant surface area for the production of digestion bacteria.
The moral of the story is that I have enough biological filtration in my 110G tank to suffice for a tank much larger than, what I would anticipate to be, 500G.
In addition, in wet/dry filtration processes, a tank volume turnover rate of 5 times per hour is desirable for a fairly heavily stocked tank.
The pump which returns the water in the sump to the tank for my filtration produces a tank turnover volume of approximately 10 times per hour.